(1) Field of the Invention
This invention is in the field of signal processors for radar signal receiving systems, and more particularly relates to a sidelobe cancellation circuit used to suppress the effects of undesired signals being received by the main channel antenna.
(2) Description of the Prior Art
Sidelobe cancellers are not new in the field of radar signal processing; nor is multiplexing a novel technique to overcome the need for multiple parallel circuits performing essentially the same signal processing function on analogous signals. Consider, as a first example, U.S. Pat. No. 3,202,990, which issued on Aug. 24, 1965 to Inventor Paul W. Howells. Though the Howells patent was one of the early works in the field of sidelobe cancellers, Howells recognized the need for and benefits accruing from a system utilizing more than one auxiliary antenna. The Howells approach, as shown in FIG. 2 of the above noted patent, is to provide each auxiliary antenna with a full complement of processing circuitry. As a further point for future reference, note that the cancellation of undesired signals in Howells occurs at intermediate frequency (IF).
A later variation to the Howells patent appears in U.S. Pat. No. 4,044,359, in which Howells is a joint inventor with Sidney P. Applebaum and James C. Kovarik. This patent further refines the concept of sidelobe cancellation with multiple auxiliary antennas, but again adopts the signal processing structure in which each auxiliary antenna retains an independent receiver and correlator. Note again that cancellation of the undesired signals in the main channel is performed at IF.
Sidelobe cancellation at IF was the subject of another invention which issued as U.S. Pat. No. 3,982,245 on Sept. 21, 1976. Inventors Hendrick H. Soule, Jr. and John F. Jureller address themselves to a sidelobe canceller configuration in which sample and hold functions or preprogrammed weights are interposed in the correlation signal path. As was the case in the prior art, Soule, Jr. et al modifies the IF operation without deleting the multiple channels of processing or transferring the cancellation function to the radio frequency (RF) stages of circuitry.
Joseph F. Len and Peter M. Rankin, in U.S. Pat. No. 3,881,177, appear to have been the first to recognize that sidelobe signal cancellation at RF is feasible. Len et al discloses that phase and amplitude modulation and signal subtraction at RF overcome limitations in earlier techniques which transposed all signals to IF prior to such processing. Nevertheless, the benefits associated with deleting the multiplicity of auxiliary antenna receivers and correlators was not recognized, as is shown by the repetative use of such functional blocks in FIG. 2 of Len et al.